Overview of Silicon Nitride Industry Technology
Silicon nitride is a compound synthesized under artificial conditions. Although silicon nitride was synthesized directly more than 140 years ago, it was only left in people's memory as a stable "refractory" nitride. After World War II, with the rapid development of science and technology, there is an urgent need for materials with high temperature resistance, high hardness, high strength and corrosion resistance. After long-term efforts, silicon nitride was not paid attention to until 1955. It was not until the mid-1970s that silicon nitride ceramic products with high quality, low cost and wide and important applications were really made.
Silicon nitride technology has been studied in China since the mid-1980s. It mainly studies the structural silicon nitride material porous silicon nitride material with the highest weight reduction efficiency. The research on silicon nitride composites has just started. There are few relevant studies on the theoretical design and experimental design of the composition system of porous silicon nitride composites, which is still in the exploratory stage, and is affected by less relevant research data at home and abroad, In this aspect, our country's research has been in a relatively backward position, many research units and scholars focus on the field of military industry, while the applied research in other fields is basically blank. The research in this area needs to be further strengthened. The prediction of dielectric constant and its influence on properties of porous silicon nitride ceramics are not fully understood, and there are few theoretical and experimental studies.
Production Process of Silicon Nitride Products:
Silicon nitride products can be divided into reaction sintered products, hot pressed products, atmospheric pressure sintered products, isostatic pressing sintered products and reaction reburning products. Among them, reactive sintering is a common method to produce silicon nitride refractory products.
The production of silicon nitride products by reaction sintering is to grind the silicon powder (particle size is generally less than 80) μ m) The green body is formed by mechanical pressure or isostatic pressing, dried, heated to 1350 ~ 1400 ℃ in nitrogen, and nitrided at the same time in the firing process. With this production method, the raw material conditions, firing process and atmosphere conditions have a great influence on the properties of the products.
There are many impurities in silicon powder, such as Fe, CA, a, Ti, etc. Fe is considered to be the catalyst in the reaction process. It can promote the diffusion of silicon, but at the same time, it will also cause defects such as pores. The main function of Fe as an additive: it can be used as a catalyst in the reaction process to promote the formation of SiO2 oxide film on the surface of products; Iron silicon melting system is formed, and nitrogen is dissolved in liquid FeSi2 to promote β- Formation of Si3N4. However, if the iron particles are too large or too high, there will be pores and other defects in the products, which will reduce the performance. Generally, the amount of iron added is 0-5%. Impurities such as Al, Ca and Ti are easy to form eutectic with silicon. Appropriate addition can promote sintering and improve the properties of products.
The finer the particle size and the larger the specific surface area of silicon powder, the lower the sintering temperature. Compared with the silicon powder with coarse particle size, the silicon powder with fine particle size contains α- The amount of Si3N4 increased. Reducing the particle size of silicon powder can reduce the micro pore size of products. The density of the product can be increased by appropriate particle size ratio.
Temperature has a great influence on nitriding rate. The nitriding reaction begins at 970-1000 ℃, and the reaction rate increases at 1250 ℃. In the high temperature stage, because of exothermic reaction, if the temperature exceeds the melting point of silicon (1420 ℃), flowing silicon is easy to occur, which will make the silicon powder body melt and collapse seriously.
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